1. Field of the Invention
This invention relates to an electrophotographic fixing device for an electrophotographic reproduction apparatus, wherein a heat fusible developed image formed on a reproduction material is thermally fixed. In more detail, the present invention is particularly concerned with an improved electrophotographic fixing device of a heat roller type, wherein various advantages are perfectly realized, such as reduction in length of the preheating time, simplification in maintenence, miniaturization in the fixing device, stabilization in image-fixing and forwarding of the reproduction material, and prevention of the reproduction material from wrapping around the roller.
2. Description of the Prior Art
There has heretofore been known a heat roller type fixing device of this sort, wherein one of a pair of the rollers, which contacts the side of the reproduction material bearing the developed image, is made in the form of a heat-conductive tube, in which a source of heat is provided, and the other roller is constructed in the form of a resilient roller. According to this construction of the fixing device, the heat conductivity of the roller which is at the side of the developed image is satisfactory, and its thermal fixation is found to be superior. On the other hand, however, since the surface of the developed image is press-contacted by the resilient roller against the surface of the tubular roller having a hard surface, the reproduction material is liable to deform by curving along the peripheral surface of the hard tubular roller with the consequence that the reproduction material tends to cause wrapping around the hard tubular roller along with adhesivity of the molten developing agent. This tendency has been considerably accelerated as the fusibility of the toner becomes better and better. In order, therefore, to prevent the reproduction material from sticking around the roller surface, it has become essential that, on the surface of the roller which contacts the developed image side of the reproduction material, an offset prevention liquid for the developed image be applied to prevent the reproduction material from sticking around the roller. However, since most of this offset prevention liquid is usually fatty in nature, and is therefore liable to contaminate the reproduction material upon its adhesion thereto, there have been brought about grave obstructions such as unsmooth hand-touch on the surface of the reproduction material after completion of the reproduction operation, incompatability of the surface of the reproduction material with ink, when manuscript in ink becomes necessary thereon, contamination of air due to evaporation of the offset prevention liquid, and so on.
On the other hand, it has been contemplated that a resilient roller be used as the roller to contact the developed image side of the reproduction material so as to avoid wrapping of the reproduction material around the roller, from which a favorable result has been obtained. In this case, however, as the heat source is placed within the resilient tubular body which is usually inferior in its heat conductivity, a considerably long time is taken to obtain the surface temperature of the roller required for the thermal fusion of the toner, which has been the cause for the long waiting time until the reproduction operation starts, hence this does not satisfy the practical requirements.